Field of the Invention
The invention relates to the field of electronic reproduction technology and pertains to a method for the transformation of color values that have been produced for a first printing process into the color values of a second printing process so that the visual impression of the colors is the same in both printing processes, the intention being for black/white image information that is printed only with the black printing ink in the first printing process also to be printed substantially only with the black printing ink in the second printing process.
In reproduction technology, printing originals for printing pages are produced that contain all the elements to be printed, such as texts, graphics, and images. In the case of the electronic production of the printing originals, these elements are present in the form of digital data. For an image, the data is generated, for example, by the image being scanned point-by-point and line-by-line in a scanner, each image point being broken down into color components and the color components being digitized. Images are usually broken down in a scanner into the color components red, green, and blue [R,G,B], that is to say, into the components of a three-dimensional color space. However, other color components are needed for color printing. In four-color printing, these are the printing inks cyan, magenta, yellow, and black [C,M,Y,K], that is to say, the components of a four-dimensional color space. For such a purpose, the image data from the RGB color space of the scanner must be transformed into the CMYK color space of the printing process to be used.
Such color space transformations are needed in reproduction technology because all the devices and processes have their restrictions and special features in the representation and reproduction of the colors, and all the devices and processes have different such characteristics. There are, therefore, different color spaces for various devices and processes, such as scanners, monitors, proof output devices, printing processes, and so on, the color spaces in each case describing the color characteristics of the device or process in an optimum way and being called device dependent color spaces.
In addition to the device dependent color spaces, there are also device independent color spaces that are based on the human visual characteristics of what is referred to as a standard observer. Such color spaces are, for example, the CIE-XYZ color space defined by the Commission Internationale d'Éclairage (CIE) standardization commission or the LAB color space derived therefrom, the LAB color space having become more widespread in the technology. If it is desired to know whether two colors will be perceived as the same or different by the human eye, then the measurement of the CIE-XYZ or LAB color components is sufficient. The LAB color components form a color space with a lightness axis [L] and two color axes [A,B], which can be imagined in the plane of a color circle through whose center the lightness axis runs. The LAB color components are related to the CIE-XYZ color components through nonlinear conversion equations.
A device or a process can be characterized with regard to its color characteristics by all the possible value combinations of the associated device dependent color space being assigned the LAB color components that a human will see in the colors produced by these value combinations. For a printing process, the various CMYK value combinations in each case produce a different printed color. Using a calorimeter, it is possible to determine the LAB components of the printed colors and to assign them to the CMYK value combinations. Such an assignment, which sets the device dependent colors produced by a device or a process in a relationship with a device independent color space (CIE-XYZ or LAB) is also called a color profile, an output color profile in the case of a printing process. The definition and data format for color profiles have been standardized by the International Color Consortium (ICC)—Specification ICC.1:1998-09. In an ICC color profile, the assignment of the color spaces in both directions is stored, for example, the assignment LAB=f1 (CMYK) and the inverted assignment CMYK=f2 (LAB).
The assignment defined by a color profile can be implemented with the aid of a table memory. If, for example, the CMYK color components of a printing process are to be assigned the LAB color components, the table memory must have a memory location for each possible value combination of the CMYK color components, in which location the associated LAB color components are stored. However, such a simple assignment method has a disadvantage that the table memory can become very large because of the large number of possible value combinations. To reduce the size of the table memory, therefore, a combination of table memory and interpolation methods is used to describe a color profile and to implement an appropriate color space transformation. The table memory is not used to store the assignments for all the possible value combinations of the CMYK color components but only for a coarser, regular grid of reference points in the CMYK color space, for example, for a grid having 16×16×16×16=65,536 grid points. For each grid point, the associated components of the LAB color space are stored as reference points in the table memory. For CMYK value combinations that lie between the grid points, the LAB values to be assigned are interpolated from the adjacent reference points. For the inverted assignment CMYK=f2 (LAB), for example, a grid of 16×16×16=4096 grid points is formed in the LAB color space, and the associated CMYK values are stored as reference points in the table memory.
The assignments given in the color profiles between device dependent color spaces and a device independent color space (for example, LAB) can be used for color space transformation between the device dependent color spaces so that, for example, the color values [C1, M1, Y1, K1] of a first printing process are converted into the color values [C2, M2, Y2, K2] of a second printing process such that, according to the visual impression, the second print has the same colors as the first print. Such a color space transformation is required if a printed page that is to be printed later by offset printing, for example, is to be output firstly on a proof printer, for example, an inkjet printer, the intention being for the colors to be reproduced in exactly the same way as they will appear in the result of the offset printing.
FIGS. 1A and 1B show a color space transformation for such a printing process adaptation according to the prior art in a block diagram. In FIG. 1A, a first color space transformation 1 from the color values [C1, M1, Y1, K1] of the first printing process into LAB color values, and a second color space transformation 2 from the LAB color values into the color values [C2, M2, Y2, K2] of the second printing process are listed one after the other. The two color space transformations 1 and 2 can also be multiplied together to form a linked color space transformation 3, which assigns the color values [C1, M1, Y1, K1] and the color values [C2, M2, Y2, K2] to one another directly (FIG. 1B). Because, through the device independent LAB intermediate color space, in each case the color values [C1, M1, Y1, K1] and [C2, M2, Y2, K2] that result in the same LAB color values are assigned to one another, the associated printing colors in the two printing processes will be perceived as visually identical within the printing color gamut.
However, one disadvantage with such a method is that what is called the black build-up of the first printing process is lost. Black build-up is understood to be the composition of printed colors with respect to their proportion of the black printing ink K. The intention, in particular, is for purely black colors, such as texts or black/white images, which are built up only with the printing ink K1 in the first printing process, that is to say, contain no [C1, M1, Y1] components, to be built up virtually exclusively with the printing ink K2 in the second printing process as well. Small proportions of [C2, M2, Y2] can be added to simulate in the second printing process the color of the paper used in the first printing process. This cannot be achieved with the method according to the prior art described because, in the intermediate color space LAB, it is no longer possible to determine whether a gray color was originally built up only with a proportion of K1 and without [C1, M1, Y1] components, or whether it was composed of large proportions of [C1, M1, Y1] and only a small K1 component. In general, based upon visual equality, that is to say, the same LAB color values, mixed colors are assigned in the second printing process, in which the gray colors also contain considerable proportions of [C2, M2, Y2]. This leads to black texts and strokes being given colored edges after the printing process adaptation in the event of register errors in the proof printer. A further problem is that printing process fluctuations of the proof print in the colored inks [C2, M2, Y2] immediately lead to clearly visible color casts in the black/white images. In addition, slight deviations from the gray axis, which can occur as a result of unavoidable residual inaccuracies in the four-dimensional transformation tables, are particularly striking in the black/white images.
In European Patent Application 0 898 417 A2, a solution is described that assigns to a color in the first printing process that is built up only from the black printing ink, that is to say, [C1=0, M1=0, Y1=0, K1], a color in the second printing process that is also built up only from the black printing ink, that is to say, [C2=0, M2=0, Y2=0, K2]. For such a purpose, in the first color space transformation 1 from the color values [C1, M1, Y1, K1] of the first printing process into LAB color values, the colors with the characteristic [C1=0, M1=0, Y1=0, K1] are assigned an extreme marginal area in the LAB color space that is not occupied by the chromatic colors of a natural image. In the second color space transformation 2 from the LAB color values into the color values [C2, M2, Y2, K2] of the second printing process, such a marginal area in the LAB color space is, in turn, assigned colors with the characteristic [C2=0, M2=0, Y2=0, K2]. Such a method carries the disadvantage that these colors in the LAB color space are not depicted in the way in which they are perceived visually. In addition, the method takes no account of the possibly different paper colors in the two printing processes, which may make it necessary for the pure gray colors of the first printing process, nevertheless, not to be built up purely from the black printing ink in the second printing process.